There has been known a tandem-type image-forming device including photosensitive drums for respective colors yellow, magenta, cyan, and black, wherein the photosensitive drums are arranged in a line. This type of image-forming device includes developing rollers for supplying toner of respective colors to the surfaces of the respective photosensitive drums. Supplying toner onto the surfaces of the photosensitive drums form toner images of respective colors substantially simultaneously.
The toner images on the surfaces of the respective photosensitive drums are directly transferred onto a sheet of paper with the images superimposed one on the other, forming a full-color image. Alternatively, the toner images are once transferred onto an intermediate transfer belt, forming a full-color image thereon, and the full-color image is transferred from the intermediate transfer belt onto a sheet of paper. In this manner, a full-color image is formed on a sheet of paper.
In one type of such a tandem-type image-forming device, each of the developing rollers is disposed to be capable of contacting and separating from the corresponding photosensitive drum.
For example, in an image-forming device proposed in Unexamined Patent-Application Publication No. 2002-6716, a lever is provided for each of developing units. By pressing the developing unit with the corresponding lever, a developing roller provided in the developing unit is separated from the corresponding photosensitive drum. By releasing the pressing, the developing roller is brought into contact with the corresponding photosensitive drum.
In an image-forming device proposed in Unexamined Patent-Application Publication No. 2004-301899, photosensitive drums and corresponding developing units are arranged in the vertical direction. A separating lever having a branch which gets under the corresponding developing unit is provided so as to be movable in the vertical direction. A developing roller provided in the developing unit is separated from the corresponding photosensitive drum by moving the separating lever upward to raise the corresponding developing unit with the branch and then rotating the developing unit about a rotational shaft thereof. On the other hand, the developing roller is brought into contact with the corresponding photosensitive drum by moving the separating lever downward to separate the branch from the developing unit and then rotating the developing unit about the rotational shaft (in a direction opposite to the direction to separate the developing roller from the photosensitive drum).
However, with the configuration disclosed in the Unexamined Patent-Application Publication No. 2002-6716, levers are required one for each developing unit, resulting in the increase in the number of components.
With the configuration disclosed in the Unexamined Patent-Application Publication No. 2004-301899, since the distance of separation between the photosensitive drum and the developing roller depends on the distance between the rotational shaft and the developing roller as well as the moving amount of the separating lever, variations in the distance of separation is likely to occur among the developing rollers. Especially when the branch of the separating lever is bent due to the weight of the developing unit, the distance of separation of the developing roller in the developing unit will greatly differ from that of the other developing rollers.